Removing nitrogen oxides (hereinafter referred to as NO.sub.x) from exhaust gases, such as combustion exhaust gases resulting from combustion of fuels and reaction exhaust gases resulting from chemical reactions is one of the major social requirements at present with a view to preventing environmental pollution.
One of the conventional methods of removing NO.sub.x from exhaust gases by reduction is known to use platinum, cobalt oxide, oxides of iron, etc. as reduction catalyzer and reduce NO.sub.x contained in exhaust gases to harmless N.sub.2 and H.sub.2 O in the presence of ammonia.
Platinum and cobalt oxides are however very expensive. Besides, in the conventional method mentioned above, catalyzers are commonly used in the form of composite catalyzer supported by a carrier. For example, when iron oxides of iron are used as catalyzer, an iron salt supported by a carrier must be oxidized or thermally decomposed and aided by a promoter, thus requiring much labor and cost. Moreover, the average NO.sub.x removal rate obtained from tests carried out with a synthetic gas consisting of NO.sub.x, NH.sub.3 and N.sub.2 in accordance with the conventional method is, at 13,000SV (SV is an abbreviation of space velocity and is given by a quotient obtained by dividing the flow rate of gas per hour (Xm.sup.3) by the quantity of catalyzer (Ym.sup.3), is about 85% for platinum catalyzer at about 220.degree. C., about 35% for cobalt oxide catalyzer at about 500.degree. C. and about 55% for oxides of iron catalyzer at about 450.degree. C. However, when used as catalyzer for removing NO.sub.x from actual industrial exhaust gases, especially exhaust gases containing SO.sub.x gases such as those from sintering plants and combustion exhaust gases from boilers, such catalyzers as mentioned above have short catalyst life and the NO.sub.x removal rate decreases relatively soon.
Besides, the catalyzers must be regenerated and replaced by new ones when their NO.sub.x removal rate decreases to below a certain limit, thus requiring much labor and cost. However, to discard used catalyzers of these types is uneconomical and may cause environmental pollution. Moreover, it is difficult to use these catalyzers after service for other purposes without additional treatment.
On the other hand, powder and/or fine particles (hereinafter referred to as said particles) containing mainly oxides of iron such as FeO, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and a mixture thereof, which are used or generated in ironworks, steelworks, etc. (hereinafter referred to as said ironworks) are a great variety of materials in large quantities, e.g., iron ore, fine ore, e.g., bedding ore for sintering, mill scale, scale of ferroalloys of Si, Mn, Ni, Cr and others as addition agents, iron-containing dust generated and collected in said ironworks, etc. Said particles are generally subjected to such processes as milling, pelletizing, sizing and classifying prior to the use as ironmaking material or as material for iron-containing products, e.g., pigment, and other effective applications of said particles are not well known.
We have attained this invention by finding out that said particles can be used as reduction catalyzers of NO.sub.x contained in exhaust gases in the presence of ammonia substantially with no additional treatment, as a result of years of studies on effective utilization of said particles containing mainly oxides of iron, which are used or generated in said ironworks, on the one hand and on reduction catalyzers of NO.sub.x contained in industrial exhaust gases on the other hand.